The present invention relates generally to fluid bed boilers, particularly improved fluidizing nozzle or bubble cap assemblies for air distribution grids in fluid bed boilers.
An air distribution grid is an important feature of a fluid bed boiler. Its purpose is to achieve a uniform air distribution across the bed plan area to fluidize the bed material in the furnace and to prevent backsifting of the bed material into the windbox. The most typical air distribution grid design is an array of bubble cap assemblies attached to a water-cooled membrane panel. Designs of bubble cap assemblies vary widely; two examples are shown in FIG. 1 and FIG. 2. A bubble cap assembly comprises bubble cap 1 and stem 2 that connects the cap 1 to an opening 3 in membrane 4 which is welded to water-cooled tubes 5.
During a start-up, if the boiler uses in-duct start-up burners, the air distribution grid is subjected to hot gases with a temperature that can exceed 1600° F. The bubble cap assemblies (typically made of stainless steel) have essentially the same temperature as these gases. Membrane 4, welded to tubes 5 and protected from direct contact with the hot gases by refractory 10 in the design shown in FIG. 2, would have a temperature close to the saturation water temperature in tubes 5, i.e. somewhere from 500° F. to 650° F., depending on the drum pressure. Membrane 4 is typically made of carbon steel. Welding stems 2, typically made of stainless steel, to the carbon steel membrane 4 creates dissimilar metal welds where the material with a higher thermal expansion coefficient (stainless steel) is at a much higher temperature than the material with a lower thermal expansion coefficient (carbon steel) thus resulting in high thermal stresses and a corresponding potential for cracking.
In order to avoid the weld cracking, the design shown in FIG. 2 features tack welding 15 of stem 2 to membrane 4, allowing their independent thermal expansions. Accommodating these expansions during start-up requires a gap 20 between the outside of the stem and the inside of the opening in the membrane 4. The stems' expansion at start-up and contraction at normal operation (when the stem temperature is somewhere from 300° F. to 500° F., depending on the temperature of the air flow through the stems at normal operation) results in a gap 25 between stem 2 and refractory 10. Therefore, the design shown in FIG. 2 is prone to air leakage through these gaps, with the leakage air bypassing the bubble caps 1. Lowering air flow through the bubble caps 1 leads to lowering the pressure drop across the bubble caps 1; this is conducive to bed material backsifting through the bubble caps 1 into the windbox. The backsifting can also result in plugging and erosion of the bubble caps 1.
Thus, there is a need for a system which avoids weld cracking. A system not prone to air leakage is also needed, so as to avoid the resultant lowering of pressure drop across the bubble caps, and reduce the potential for bed material backsifting as well as plugging and erosion of the bubble caps.